/*************************************************************************
	> File Name: oj-394_jump_rock.cpp
	> Author: Wang Bo
	> Mail: 1276106221@qq.com
	> Created Time: 2020年11月01日 星期日 20时25分26秒
 ************************************************************************/

#include <iostream>
using namespace std;

int ll, n, m, num[50005], tl;

int func(int d) {
    int cnt = 0, last = 0;
    for (int i = 1; i <= n + 1; ++i) {
        if (num[i] - last < d) {
            cnt++;
        }
        else {
            last = num[i];
        }
    }
    return cnt;
}

int bin_search() {
    int l = tl, r = ll;
    while (l != r) {
        int mid = (l + r + 1) / 2;
        int s = func(mid);
        if (s <= m) l = mid;
        else r = mid - 1;
    }
    return r;
}

int main() {
    cin >> ll >> n >> m;
    for (int i = 1; i <= n; ++i) {
        cin >> num[i];
        if (i == 1) {
            tl = num[1] - num[0];
        }
        else {
            //更新最小值
            tl = min(tl, num[i] - num[i - 1]);
        }
    }
    num[n + 1] = ll;
    cout << bin_search() << endl;
    return 0;
}
